1. Field of the Invention
This invention relates broadly to airfoil structures. More particularly, this invention relates to airfoil structures employing molded materials for components of the airfoil structure.
2. State of the Art
An airfoil is a body designed to obtain reaction upon its surface from air through which it moves or from air that moves past it. Airfoils are used in many applications, such as aircraft wings, ailerons, stabilizers, wind turbine blades (which generally include two types, horizontal axis blades and vertical axis blades), helicopter rotors, propellers, fan blades, etc. Airfoils include a characteristic cross-sectional shape as shown in FIG. 1, including a rounded leading edge, a trailing edge, and top and bottom skins defining an upper chamber section and lower chamber section over a cord extending between the leading edge and trailing edge. The asymmetry between the top and bottom skins of the airfoil produces aerodynamic forces acting on the top and bottom skins as air moves past these surfaces, even at a zero angle of attack. For aircraft wings and the like, these aerodynamic forces generate lift that is used to fly the aircraft. For helicopter rotors, propellers and the like, these aerodynamic forces generate thrust that is used to propel and fly the aircraft. For wind turbine blades, these aerodynamic forces cause rotation of the blade for driving a turbine that converts mechanical rotation of the blade into electrical power. For most applications, the cross-sectional shape of the airfoil tapers over a spanwise direction from root to tip from a relatively thick cross-section at the root to a relatively thin cross-section at the tip. Moreover, for some applications, such as wind turbine blades, the cross-sectional shape of the airfoil can bow, twist and/or curve over the spanwise direction to accommodate centrifugal forces and/or optimize the angle of attack of the blade relative to the wind in order to smooth rotational speed of the blade and reduce pulses that are common in vertical-axis wind turbine designs.
Airfoils generally include one or more solid web elongate spars that extend along the spanwise direction of the airfoil and can also include a plurality of transverse ribs spaced apart along the spanwise direction that define the cross-sectional shape of the air foil. Top and bottom skins are fixed to the spars and ribs and make up the aerodynamic outer surface of the airfoil. Coatings and/or paints can be applied to the skins in some applications. The spar(s), ribs and skins resist the shear, compression, bending and buckling, and tensional and tensional twist loads applied to the airfoil during use. The spar(s) primarily resist bending and buckling loads and the shear loads that result therefrom. The ribs primarily resist tensional twist loads. And the skins act as flanges that resist bending and buckle loading, compression loading, as well as tensional and tensional twist loading. Moreover, the skin can experience shear stress from bending and buckling, compression and tensional and tensional twist loads. Such shear stress can lead to failure of the skin (typically involving delamination of the skin and possibly loss of skin sections from the airfoil), which can potentially compromise the structural integrity of the airfoil.
Many contemporary airfoils are hollow. In such hollow designs, all of the forces acting on the airfoil (including forces that result from bending and buckling, compression and tensional and tensional loading) are channeled through the skin, which significantly increases the shear stresses imparted on the skin during use. Hereto, such shear stress can lead to failure of the skin (typically involving delamination of the skin and possibly loss of skin sections from the airfoil), which can potentially compromise the structural integrity of the airfoil.
As an airfoil increases in size, the bending, buckling, tensional and tensional twist loads imparted on the airfoil increase. Such increased loading requires skin and/or spar designs that utilize more material in an effort to ensure the structural integrity of the airfoil. At the same time, larger dimensions and weight reduce the strength-to-weight ratio of the airfoil. For aircraft wings and helicopter rotors, the increase in weight of the airfoil increases the power required to move the airfoil and thus reduces fuel efficiency. For wind turbine blades, the increase in weight of the airfoil significantly decreases the generating efficiency of the wind turbine system.